Chemical Properties
colorless, rhomb crystal(s) or white, crystal(s) powder(s); darkens on exposure to light; it is a moderately strong acid; used in analytical chemistry and in medicine [HAW93] [MER06]
Physical properties
Iodic acid, HIO₃, can be obtained as a white solid. It dissolves readily in water, but it can also exist in the pure state, unlike chloric acid or bromic acid. Iodic acid contains iodine in the +5 oxidation state and is one of the most stable oxoacids of the halogens in its pure state. When iodic acid is carefully heated, it dehydrates to iodine pentoxide. Upon further heating, iodine pentoxide decomposes further, producing a mixture of iodine, oxygen, and lower oxides of iodine.
Uses
Iodic acid is used in analytical chemistry laboratories to standardize solutions of both weak and strong bases using methyl red or methyl orange as the indicator. It acts as a key starting material to synthesize sodium or potassium iodate, thereby increasing the iodine content of salt.
Preparation
Iodic acid may be prepared by the reaction of sulfuric acid with bariumiodate. The solution is filtered to remove barium sulfate and then crystallizedto obtain iodic acid:
Ba(IO
3)
2 + H2SO
4 → BaSO
4 + 2HIO
3
It also may be produced by oxidation of iodine with concentrated nitric acid:
3I
2 + 10HNO
3 → 6HIO
3 + 10NO + 2H
2O
Also, iodic acid may be obtained by oxidation of iodine with chlorine in diluteacidic solutions:
I
2 + 5Cl
2 + 6H
2O → 2HIO
3 + 10HCl
Another method of preparation involves oxidation of iodine with hydrogenperoxide:
I
2 + 5H
2O
2 → 2HIO
3 + 4H
2O
It also may be prepared by treating hypoiodous acid with a base:
3HIO + 2OH¯ → HIO
3 + 2H
2O + I¯
Hypoiodous acid may be obtained by alkaline hydrolysis of iodine at pH 12:
I
2 + H
2O → HIO + H
+ + I¯
Iodic acid dehydrates to iodine pentaoxide when heated at 180°C:
2HIO
3 → I2H
5 + H
2O
Iodic acid is a relatively weak monoprotic acid, the Ka value at 25°C is 1.6x10-1. Several species have been detected in concentrated aqueous solutions,which include IO3
-, H
+, HIO
3, (HIO
3)
2 and (HIO
3)
3. Its solution turns blue lit-mus red and then bleaches the litmus paper because of its strong oxidizingproperties.
When heated with potassium iodate, potassium hydrogen iodate is formed:
HIO3 + KIO3 → KH(IO3)2
An aqueous solution of iodic acid is a strong oxidizing agent. It liberates iodine from iodides:
IO
3¯ + 5I¯ + 6H
+ → 3I
2 + 3H
2O or, HIO
3 + 5HI → 3I2 + 3H
2O
In an aqueous solution, iodic acid oxidizes hydrogen sulfide to sulfur:
2HIO
3 + 5H
2S → I
2 + 6H
2O + 5S
The solid iodic acid reacts vigorously with sulfur, phosphorus and other non-metals.
Definition
Iodic acid: Any of various oxoacids of iodine, such as iodic(V) acid and iodic(VII) acid. When used without an oxidation state specified, the term usually refers to iodic(V) acid (HIO₃). It has a role as an astringent. It is a conjugate acid of an iodate. Aqueous solutions of iodic acid serve as strong oxidizing agents. The acid is also used in redox titrations.
General Description
Iodic acid is the hydrated form of I
2O
5. Reaction of iodic acid with hydrogen iodide has been described by electrolytic dissociation theory. Combustion of mixture of chromic, iodic, sulfuric and phosphoric acids has been proposed. Its Raman spectra have been recorded. Vibrational assignment of IO
3- has been evaluated.
Hazard
Toxic by ingestion, strong irritant to eyes
and skin.
Purification Methods
Dissolve iodic acid in the minimum volume of hot dilute HNO3, filter and evaporate in a vacuum desiccator until crystals are formed. Collect the crystals and wash them with a little cold H2O and dry them in air in the dark. It is soluble in H2O: 269g/100mL at 20o and 295g/100mL at 40o. It is soluble in dilute EtOH and darkens on exposure to light. It is converted to HIO3.I2O5 on heating at 70o, but at 220o complete conversion to HIO3 occurs. [Lamb et al. J Am Chem Soc 42 1636 1920, Bray & Caulkins J Am Chem Soc 53 44 1931.]
